1. Field of the Invention
The present invention is generally related to offshore platforms and particularly to offshore platforms where a vertical restoring couple is used to counter platform sway.
2. General Background
Most offshore oil and gas production is conducted from platforms secured to the ocean bottom. A key design constraint for such platforms is that there be no substantial dynamic amplification of the platform's response to waves. This is accomplished by designing the platform to have natural vibrational periods which do not fall within that portion of the range of wave periods representing waves of significant energy. The several modes of platform vibration which are generally of greatest concern in platform design are pivoting of the structure about the base(commonly termed "sway"), flexure(bending) in the vertical plane, and torsion about the vertical axis. For deep water applications, greater than about four hundred meters, the conventional rigid structure design becomes uneconomical. It then becomes necessary to use compliant platforms that have a sway period greater than the range of periods of ocean waves containing significant energy. A compliant platform uses its own inertia to increase the sway period, thereby reducing the dynamic amplification of the platforms response to waves, which in turn reduces the structural steel needed and higher cost associated with a given increase in water depth. Compliant platforms that use a guyed tower present the problems of high cost and interference with navigation and fishing in the area of the platform. The use of positive buoyancy by a tension leg platform presents problems of greater complexity and large and expensive hull structures.
U.S. Pat. No. 4,696,603 discloses a compliant offshore platform in which a number of flex piles that pass through guides are driven into the sea bottom adjacent each corner of the space frame structure of the offshore platform. The flex piles extend upward to a flex pile connector at a preselected location on the space frame structure where they are connected to the space frame structure. The flex piles act to maintain the platform sway within the desired vibration period range by being forced into tension and compression in conjunction with the sway of the platform.
Such a compliant platform presents certain difficulties and problems in transportation and installation. The space frame structure, commonly referred to as a tower, is very slender, i.e., it has a small cross sectional dimension at its base relative to its height. As such, it is susceptible to overturning during installation from forces generated by storms or by deep water currents. Until the requisite minimum number of piles are driven and attached to the tower, the tower is at greater risk of damage or loss from overturning than a conventional fixed platform would be. For these same reasons, water currents will easily tilt the tower from plumb prior to attachment of the flex piles to the tower. The flex piles are extremely long. The assembled length, including penetration into the sea bottom will exceed 1,500 feet in most cases. The longest piling that applicant is aware of that has been handled in one piece has been about 500 feet in total length. Thus, the flex piles will require specialized techniques offshore to assemble from sections short enough to be handled successfully. The tower must be built on shore lying on one side, skidded onto a launch barge, towed to the site, and launched into the water. Where the tower is sufficiently narrow, the corner legs of the tower can be situated above the launching ways of the launch barge. For this case, the tower will be supported on its side by the two lower corner legs, which rest in structural plate assemblages called cradles, which in turn rest on the launch ways of the launch barge. However, there is an array of guides for the flex piles about each corner leg at various positions along the legs. Therefore, the geometry of the array and the depth of the cradles must be chosen so that features on the tower, notably the flex pile guides, and features on the launch barge do not interfere. This problem results in deep cradles and, therefore represent a significant expense. Since the flex piles are immersed in sea water from the mud line upward, they must be protected from corrosion by sacrificial anodes. Because of the large surface area, the cost of the anodes required is significant. The connection of the flex piling to the tower at the top end of the flex piles and the framing members that attach the flex pile guides to the tower at various elevations along the piling represent a significant expense. Horizontal translation of the tower at the mud line is resisted by the flex piles acting in shear. In order to develop the shear in the flex piles, the bottom two levels of flex pile guides must develop a horizontal couple. This presents a wear problem when the force from the horizontal couple presses the outside of the flex pile against the inside of the flex pile guide while the flex pile is simultaneously moved axially relative to the flex pile guide by the compliant action of the platform. The resultant rubbing causes some wear from friction but results in an accelerated corrosion rate because the iron oxide and calcium carbonate coating being formed is constantly being rubbed off and exposes fresh elemental iron to the corrosion process. This can not readily be compensated for bey thickening of the flex pile since it is not known in advance if the design penetration of the flex pile into the sea bed will actually be achieved.
Article OTC 6351 by Deserts and Cortez discloses a similar compliant tower to that above with the difference being that two structural elements are used to perform the dual functions of the flex piling. A shallow foundation template is placed on the sea floor. Foundation piling is driven through sleeves in the foundation template, the foundation template is leveled, and then the annuli between the piling and sleeves are grouted to permanently connect the foundation template to the piling. Two flex tubes are preinstalled on opposite sides of each corner leg and extend a certain distance below the bottom of the tower. Sockets in the foundation template receive the flex tube extensions. This concept avoids the problems associated with assembling and driving the long flex piles offshore. Also, the problem of on-bottom stability of the slender tower during flex pile installation is eliminated by the preinstalled foundation template. Leveling the foundation template prior to grouting eliminates the problem of plumbing the slender towers. The wear problem is greatly reduced because the flex tube is preinstalled, not driven, so the location of the wearing surfaces on the flex tubes are known and thickened walls and hardened surfaces can be provided at these locations, as required.